A publishing house’s main purpose is to find authors and their manuscripts to produce and publish into books.
They can be responsible for several things:
Selecting marketable manuscripts. Publishing houses need to know trends, statistics, and market climates to choose the right books and frame them in the best way to generate the most sales.
Designing and editing the manuscripts. As I said, publishers are responsible for framing. This also requires a knowledge of trends and market climates to design a book that will grab readers’ interest.
Selling and promoting. The publishing house makes deals with retailers and companies for when, where, and how to sell the book. They also plan and execute marketing and promotional plans, if any, for the book. This isn’t something they do for every author. Publishing houses benefit from pushing their high sales authors–very little marketing budget goes toward newbie and debut authors, so if you’re into the idea of traditional publishing because you think they’ll spend their marketing budget on you: reconsider.